The present invention relates generally to extruded PTFE products. More particularly the present invention relates to expanded PTFE products formed from an extrusion process, such products being useful in grafts, patches, tubing and the like specifically in medical applications.
The use of products formed of polytetrafluoroethylene (PTFE) in medical applications is well known. Products such as implantable grafts, implantable patches, catheter tubing and the like may be derived from extruded tubing of PTFE.
PTFE tubing is normally manufactured by a paste extrusion process. Screw injection extrusion which is typical of most thermoplastics may not be effectively used with PTFE because PTFE resin does not exhibit sufficient fluidity even when heated. In the paste extrusion process a xe2x80x9cgreen tubexe2x80x9d is formed. A green tube is a tube of PTFE that must be subjected to secondary operations before it yields a usable medical product. Such secondary operations may include stretching and expanding the tube under various conditions of time, pressure and temperature. The paste extrusion process tends to produce a tube which has a fibrous state where its fibrils are generally longitudinally aligned in the direction of extrusion. This fibrous state formation is particularly evident where the PTFE paste includes a lubricant to assist in extrusion. Extruded tubes having fibrils longitudinally aligned in this fashion exhibit low radial or hoop strength. Such a tube is highly susceptible to tearing or rupturing.
Attempts have been made to modify the structure of extruded PTFE tubing. Such attempts seek to manufacture extruded PTFE tubing having non-longitudinally aligned fibrils where the fibrous state formation includes fibrils aligned transversely to the extrusion direction. One attempt in the vascular graft art is shown in U.S. Pat. No. 4,743,480. This technique employs a screw tip on the extrusion mold to reorient the fibrils during the paste extrusion process. The pitch of the screw tip tends to twist the fibrils during extrusion.
In the mechanical art area, U.S. Pat. No. 4,225,547 employs counter-rotation to manufacture pipes and wire jackets. In this example, the mandrel and the outer portion of the extrusion die are counter-rotated with respect to one another. While this tends to orient the fibrils in both the longitudinal and transverse direction, as set forth in the ""547 patent a suitable product is only obtained by establishing during extrusion a temperature gradient where the die temperature is substantially higher than the initial temperature of the paste preform entering the die apparatus.
In this process, the die is therefor heated to a temperature significantly above the initial paste temperature. As is set forth in the ""547 patent, elevating the temperature of the die over that of the incoming paste while counter rotating the die components, subjects the product to thermal expansion and enhances the fibrous-state formation in the direction perpendicular to the direction of extrusion.
However, the process described in the ""547 patent has several disadvantages. First, it is difficult to maintain predictable processing parameters where a temperature gradient is relied upon. Further, it is difficult to maintain an environment where a temperature gradient must be established and maintained. In addition frictional heating of the paste due to contact with rotational members precludes establishment of a reproducible steady state extrusion condition where a fixed temperature gradient must be maintained. Finally, the compressible nature of PTFE pastes, coupled with their high coefficient of expansion make operation under a fixed temperature gradient highly undesirable.
It is therefore desirable to provide a process for producing a PTFE tube where fibrous-state formation is enhanced thereby resulting in a tube having higher radial is strength, without the need to maintain a precise temperature gradient during processing.
It is an object of the present invention to provide an improved method and apparatus for forming a PTFE tubular product in a paste extrusion process.
It is a further object of the present invention to provide an expanded PTFE product (ePTFE) formed by an improved process which exhibits high radial tear strength.
It is a still further object of the present invention to provide an ePTFE product having a microporous structure with substantially randomly tilted fibril and node structure.
In the efficient attainment of these and other objects the present invention provides an improved method and apparatus for forming a PTFE tubular product. The present invention provides for the extrusion of a PTFE green tube between at least one rotating die of an extrusion apparatus. The extrusion apparatus is maintained at a constant temperature to avoid a temperature gradient during extrusion. The die components may be rotated during extrusion to enhance fibrous state formation of the tube in a direction generally perpendicular to the extrusion direction. The green tube so formed may be then subjected to secondary operations such as stretching and expanding to form an ePTFE tube suitable for medical use. The ePTFE tube exhibits a microporous structure defined by nodes interconnected by elongate fibrils. The nodes in such a microporous structure are oriented such that their primary axes are not generally perpendicular to the longitudinal axis of the tubular body.